The present invention relates generally to surgical procedures for spinal stabilization and more specifically to instrumentation and techniques for inserting a spinal implant within the intervertebral disc space between adjacent vertebra. More particularly, while aspects of the present invention may have other applications, the invention provides instruments and techniques especially suited for interbody fusion from a generally posterior approach to the spine
Various surgical methods have been devised for the implantation of fusion devices into the disc space. Both anterior and posterior surgical approaches have been used for interbody fusions. In 1956, Ralph Cloward developed a method and instrumentation for anterior spinal interbody fusion of the cervical spine. Cloward surgically removed the disc material and placed a tubular drill guide with a large foot plate and prongs over an alignment rod and then embedded the prongs into adjacent vertebrae. The drill guide served to maintain the alignment of the vertebrae and facilitated the reaming out of bone material adjacent the disc space. The reaming process created a bore to accommodate a bone dowel implant. The drill guide was thereafter removed following the reaming process to allow for the passage of the bone dowel which had an outer diameter significantly larger than the reamed bore and the inner diameter of the drill guide. The removal of the drill guide left the dowel insertion phase completely unprotected. Thus, Cloward""s method and instrumentation was designed for and limited to an anterior surgical approach and was inappropriate for a posterior application.
Furthermore, B. R. Wilterberger described in a paper entitled xe2x80x9cDowel Intervertebral Fusion as Used in Lumbar Disc Surgeryxe2x80x9d (published in The Journal of Bone and Joint Surgery, volume 39A, pgs. 234-92, 1957), the unprotected drilling of a hole from a posterior approach into the lumbar spine between the nerve roots and across the disc space, and then inserting a bone dowel into that disc space. While Wilterberger had taken the Cloward concept of circular drilling followed by dowel fusion and applied it to the lumbar spine from a posterior approach, he had not further improved the method, nor had he advanced the instrumentation to provide adequate protection for the sensitive vessels and neurological structures adjacent to the operating field.
U.S. Pat. No. 5,484,437 to Michelson discloses a technique and associated instrumentation for inserting a fusion device from a posterior surgical approach that provides greater protection for the surrounding tissues and neurological structures during the procedure. As described in more detail in the ""437 patent, the surgical technique involves the use of a distractor having a penetrating portion that urges the vertebral bodies apart to facilitate the introduction of the necessary surgical instrumentation. The ""437 patent also discloses a hollow sleeve having teeth at one end that are driven into the vertebrae adjacent the disc space created by the distractor. These teeth engage the vertebra to maintain the disc space height during subsequent steps of the procedure following removal of the distractor. In accordance with one aspect of the ""437 patent, a drill is passed through the hollow sleeve to remove portions of the disc material and vertebral bone to produce a prepared bore for insertion of the fusion device. The drill is then removed from the sleeve and the fusion device is positioned within the disc space using an insertion tool.
While the more recent techniques and instrumentation represent an advance over earlier surgical procedures for the preparation of the disc space and insertion of the fusion device, the need for improvement still remains. The present invention is directed to this need and provides convenient methods and instruments to insure safe and effective preparation of a disc space in conjunction with implant placement.
One object of the present invention is to provide an improved retractor assembly permitting variable placement of the handle with respect to a retractor blade. The retractor comprises a retractor blade, a shaft having a first portion connected to the retractor blade, and an opposite second portion pivotally connected to a handle. Preferably, the assembly further includes a locking mechanism selectively locking the handle to the second portion to limit pivotal movement of the handle in relation to the shaft.
In another aspect of the present invention, a method of dura retraction is provided for posterior access to the spine. The method comprises providing a retractor having a retractor blade pivotally connected to a handle, and the handle having a locking mechanism to selectively lock the handle to the retractor. A portion of the dura is exposed and the retractor is inserted with the handle in an insertion position and the locking mechanism in a locked position. The dura is then retracted to expose underlying spinal elements. Preferably, the locking mechanism is unlocked to allow the handle to pivot in relation to the retractor blade. In this aspect, the handle is pivoted to a holding position and locked to maintain the handle in the locked position.
Yet a further aspect of the present invention is a template for straddling the dura in a spinal surgery to facilitate marking a surgical site to gain access to the disc space in preparation for implant placement. The template comprises a body having an upper surface and a lower surface facing the dura, and an opening formed between the upper surface and the lower surface. A shaft having a first end and a second end is connected to the body and extends away from the upper surface. Preferably, a working tube is connected to the body in substantial alignment with the opening and extends from the lower surface, the tube having a first diameter. A locator extension engages the body and is spaced from the tube to provide a space for passage of the dura therebetween. The locator extension extends from the lower surface and has a second diameter that is less than the tube diameter. Optionally, the body may be formed to match the maximum area of the insertion instrumentation at the engagement with the vertebral bodies, thereby allowing marking of the bone needing removal.
Still a further object of the present invention is to provide a spinal disc space distractor assembly. Preferably, the distractor includes a tapered shaft portion. Optionally, a window may be formed through the shaft for visualization. In one form of the invention, the assembly comprises a driving portion removably coupled to a distractor tip. The driving portion is coupled to transmit rotational and longitudinal forces. Preferably the assembly includes an outer shaft having a first driving shoulder for transmitting rotational force end and an opposite second driving shoulder for receiving a rotational force. An inner shaft is slidably disposed within at least a portion of the outer shaft, the inner shaft having a first connection end and an opposite second connection end. The first connection end is disposed adjacent the first driving shoulder. The assembly further includes a distraction tip, the tip having a driving surface adapted for engagement with the first driving shoulder and a connection surface adapted for engagement with the first connection end. A handle interconnects the inner and outer shafts and maintains the tip in contact with the outer shaft. In one embodiment the outer shaft is tapered to provide greater visualization. Further, the outer shaft may have a visualization window extending there through.
It is yet a further object to provide an instrument for determining the depth and size of an opening formed between two adjacent vertebral bodies. The instrument comprises an elongated shaft and preferably a radiolucent tip attached to the shaft, the tip including at least on radiopaque marker. Preferably, the instrument includes a distal tip sized to match the diameter or shape of the opening intending to be created.
The present invention also contemplates a method of preparing a disc space and inserting an implant. The method utilizes one or more of the instruments described above to prepare the disc space for receiving an implant.
Related objects and advantages of the present invention will be apparent from the following description.